The present invention relates to a plate fin type heat exchanger to be installed in an air-conditioner.
In order to improve the heat exchange efficiency of a plate fin type heat exchanger, a plurality of portions of each plate fin are cut to a small width and raised in the shape of bridges projecting in a direction which crosses the direction of a flow of an air stream, as disclosed in Japanese Patent Publication No. 63-11597/1988, or a plurality of portions of each plate fin is cut to a small width and raised so as to form a louver each slat of which project in a direction which crosses the direction of a flow of an air stream, as disclosed in Japanese Utility model Publication No. 58-49503/1983.
When the air stream flows at right angles to the surfaces of the plate fins in the heat exchanger disclosed in the above Japanese Patent Publication No. 33-11597/1988, it passes the cut and raised portions constituting the louver elements of a larger air resistance and the non-cut base portions of a smaller air resistance alternately, so that the air resistances of all parts of the interior of the heat exchanger become substantially equal to cause the heat exchange efficiency to be improved. In, for example, a wall type air-conditioner, which is mounted on a wall surface of a room, a cross flow fan is provided at the back of a lower portion of a longitudinally elongated heat exchanger body. Therefore, the air stream flows in the diagonally downward direction in the upper half portion of the interior of the heat exchanger and passes all of the louver elements of a larger air resistance. In addition, the velocity of this air stream is originally lower than that of the air stream flowing in the lower half portion of the interior of the heat exchanger. Consequently, if the rotational speed of the cross flow fan is increased for the purpose of improving the heat exchange efficiency in the upper half portion of the heat exchanger, there is the possibility that the velocity of flow of the air stream flowing in the substantially horizontal direction in the lower half portion of the interior of the heat exchanger will increase causing noise to occur when the air stream passes through the spaces among the plate fins.
In the heat exchanger disclosed in the above Japanese Utility Model Publication No. 58-49503/1983, the root portions of the louver elements are positioned around the portions of each fin which surround the heat transfer tubes so that the greater part of the air stream passes the louver elements provided in rows over the whole width of the heat exchanger. Also, the louver elements in the outside rows which are farthest away from the center line of a row of heat transfer tubes, and the louver elements in the inside rows are divided, and the lengths of these divisional louver elements are reduced, whereby the heat transfer paths between the louver elements and the base portion of the fin are shortened to enable the heat exchange efficiency to be improved. However, in spite of the fact that the louver elements in the outside rows are farthest away from the center line of the heat transfer tubes and have a low heat transfer rate, the length thereof is larger than that of the louver elements in the inside rows, so that the heat transfer efficiency of the louver elements in the outside rows does not become sufficiently high. Therefore, when this heat exchanger is installed in a wall type air-conditioner, the air stream flows in the diagonally downward direction in the upper half portion of the interior of the heat exchanger and passes all of the louver elements of a larger air resistance, and the velocity of flow of this air stream is originally lower than that of the air stream flowing in the lower half portion of the interior of the heat exchanger, as in the heat exchanger disclosed in the above Japanese patent Publication No. 63-11597/1988. Accordingly, there is the possibility that, if the rotational speed of the cross flow fan is increased, noise will be generated.